This invention relates to converting a stereo audio signal to a signal which when reproduced appears to a listener to have a more realistic sound image in three dimensions than the original stereo signal. Such conversion is commonly referred to as stereo expansion, and will hereinafter be referred to as such.
The basic patent to stereo, GB-A-394325 (EMI) describes and claims a system for producing stereo signals wherein the relative loudness of the loudspeakers is made dependent upon the direction from which the sounds arrive at the left and right input microphones. The system incorporates sum and difference circuits, the outputs of which are coupled to respective filters. Such sum and difference circuits have subsequently been incorporated into many different types of stereo systems, especially stereo expansion systems.
There are numerous stereo widening methods which with varying degrees of success attempt to widen the stereo sound image. A common element of many of these methods is the use of such sum and difference circuits, whereby the stereo input left and right signals are added and processed in one way, and the input signals are also subtracted and processed in a different way, the two such paths being recombined to produce the converted output signals. These methods are all synthetic, in the sense that they have no basis in accurate modelling of the processing theoretically required to widen the sound image. For example U.S. Pat. No. 4,748,669 to Klayman describes a stereo enhancement system which generates sum and difference signals and circuitry, including a spectrum analyser, for selectively modifying the signals.
A better method is to use so-called Head Response Transfer Functions (HRTFs), which are filters which represent the response of an artificial or human head and ears to a sound arriving from a particular direction. By the use of HRTFs, a converter can be produced which accurately models the theoretical equations which describe the widening process. U.S. Pat. No. 5,371,799 to Lowe describes a system for e.g. a video game played on a personal computer, wherein an input audio signal is applied both to left and right HRTFs, the HRTFs being modified according to the required apparent location of the audio signal source. These methods are sometimes referred to as creating virtual speakers or sources to position two speakers apparently outside the actual physical speaker positions. The problem with these methods is that there is a tonal quality change (sometimes referred to as an equalisation change) associated with the use of HRTFs to create virtual sources or speakers. This effect is undesirable and is not acceptable in many applications.
It is possible to correct for this tonal change by equalising both the input (or output) left and right signals to compensate for the tonal change produced by the HRTFs, but if this is done, the positional accuracy of the virtual sound source, speaker or loudspeaker is impaired, particularly if the virtual speakers are widely spaced.
It is an object of the present invention to provide a system of stereo expansion without causing a tonal change.
The invention is based on the fact that a typical pair of HRTFs (near ear and far ear) have a useful property; that is, that the far ear function is diminished in amplitude much more than the near ear function, especially at higher frequencies. This means that when HRTF pairs are used to create a virtual source most of the energy from the virtual source is associated with the near ear. Thus the listener perceives a tonal quality dominated by the near ear HRTF. Use can be made of this property. Both the near and far ear HRTFs can be equalised with the inverse of the near ear HRTF, thus rendering the near ear HRTF flat. Thus the ear perceives a tonal quality dominated by the near ear HRTF, which is flat. The effect is that the overall frequency response is substantially flat, and hence the tonal quality is correct. However, if this is actually implemented, it is found that the equalisation of the far ear by the same equalisation function (inverse of near ear) impairs the localisation accuracy, and the end result is not satisfactory.
However, it has been discovered that if the far ear HRTF is NOT modified, and the near ear HRTF is equalised with its own inverse as described above, i.e. it is rendered neutral and has a flattened frequency response, the stereo expander has the benefits of an apparently flat response, i.e. no tonal change, but also has the full localisation accuracy. This is the basis of the present invention. A flat response filter is of course a straight-through connection; no filtering is actually required.
The present invention provides stereo expansion apparatus comprising first and second inputs for receiving respective left and right stereo signals, the first input being coupled through a first channel which does not significantly alter the frequency characteristics of the stereo signal to a first summing means and the first input being coupled, via a filter representing a far ear HRTF for a listener, to a second summing means, and the second input being coupled through a second channel which does not significantly alter the frequency characteristics of the stereo signal to said second summing means, and, via a filter representing a far ear HRTF for a listener, to the first summing means, the outputs of the summing means providing stereo expanded signals.
The apparatus in accordance with the invention can be used either to generate binaural signals suitable for headphone listening, or can be fed into a crosstalk canceller, such as that described in our copending application WO-A-9515069 in order to generate signals suitable for loudspeaker reproduction.
In accordance with a preferred feature of the invention, an equaliser is provided to compensate for any average deviation. Preferably an equaliser is provided which has a characteristic which equalises both left and right channels either before or after stereo expansion.
It will be understood for the purposes of this specification, that HRTF or head related transfer function is intended to mean a function representing the frequency response of a path between a source of sound and the ear of the listener, either the ear nearer the sound (near HRTF) or the ear further from the sound (far HRTF). HRTFs may be obtained by measurements on a real human head equipped with suitable microphones; alternatively they may be obtained using an artificial head means, which may be as is common a precise model of a human head and torso with microphones in the ear structures; alternatively it may be something far less precise, for example a block or sheet of wood positioned between a pair of spaced apart microphones; it might even be an electrical synthesis circuit or system which creates such functions. It will be understood HRTFs are widely publishedxe2x80x94see for examplexe2x80x94Measuring a dummy head in search of pinna cuesxe2x80x94H L Han, J. Audio Eng. Soc., January/February 1994, 42, (1/2),pp.15-36